A bed of coal contains a great amount of methane adsorbed therein and an effective utilization thereof is now underway. Methane collected from an un-mined coal bed by e.g. drainage has a relatively high methane concentration ranging from 30% to 95%. Hence, its effective utilization is relatively easy through concentration or the like. Whereas, a gas discharged through ventilation of a coal mine has only a low concentration of methane ranging from 0.1 to 1%, so that most of it is discharged into the atmosphere. Although methane is not harmful for human bodies, it provides a high global warming effect. Therefore, there is a need for reducing its discharge amount. However, when a low concentration gas such as coal mine ventilation gas is to be concentrated, this concentrating process involves therein passage of the gas through the explosion limit (from 5 to 15%) of methane. Hence, there is a significant concern about safety and the process is not feasible. For this reason, there have been proposed methods of using such gas as combustion air for a gas engine or turbine or methods of oxidization removal by such technique as catalytic oxidization (Non-Patent Documents 1, 2).
In treatment of gas containing an organic compound at a low concentration, according to a process often employed in treatment of an exhaust gas generated from an industrial process containing a volatile organic compound (VOC), an oxidization catalyst and a heat exchanger are employed in combination and preheated gas is fed to the catalyst for removing the organic compound contained in the gas through a catalytic oxidization reaction (Non-Patent Documents 3, 4). This process employs a catalyst usually comprising Pt or Pd supported on an aluminum support. The target substances to be treated by this VOC treatment process are normally compounds that can be oxidized relatively easily, such as toluene, acetone, ethyl acetate. These can be oxidized relatively easy with using the above catalyst at a low temperature of 350° C. or lower.
However, methane is the most stable compound among hydrocarbons. So, with the above-described catalysts, oxidization removal of methane is difficult at low temperatures of 400° C. or lower. For instance, in Non-Patent Document 2, it is shown that satisfactory methane removal performance cannot be obtained unless the methane concentration is at least about 0.3% and even in the case of methane concentration of 0.423%, satisfactory performance cannot be obtained unless the catalyst inlet temperature is 490° C. or higher. In order to preheat coal mine ventilation gas at ambient temperature and in a great amount thereof, a heat exchanger with a large capacity is required, so there arises the problem of inferior economic performance. Further, in case the catalyst inlet temperature is about 500° C., with addition of reaction heat generated from oxidization of methane, the catalyst outlet temperature becomes from 600° C. to 700° C. This is not only detrimental to the durability of catalyst, but also causes another problem of additional costs of piping and the heat exchanger due to heat resistant temperature requirement.
Due to sulfur compounds present in coal, coal mine ventilation gas contains trace amounts of sulfur compounds (hydrosulfide, methyl mercaptan, dimethyl sulfide, sulfur oxide, etc.) These are strong catalyst poisons, which makes catalytic oxidization of methane at low temperatures even more difficult. For instance, Lee et al, studied the effect of hydrogen sulfides on methane oxidation using a Pd catalyst and revealed that in coexistence of hydrogen sulfide of 26 ppm, 50% methane removal temperature rises as much as 200° C. or more, from 360° C. to 580° C. (Non-Patent Document 5).
As oxidization removal catalysts for methane contained in combustion exhaust gas, there are known a catalyst comprising iridium and platinum supported on a zirconia support and a catalyst comprising iridium and platinum supported on a titania support (Patent Documents 1, 2). With these catalysts, oxidization removal of methane is possible at a relatively low temperature ranging from 350 to 400° C. approximately, even in the coexistence of sulfur dioxide, in addition to high concentration of steam. However, the coal mine ventilation gas treatment with these catalysts suffer the following shortcomings.
Firstly, there is a need for ensuring resistance of the catalyst against reducing sulfur compounds such as hydrogen sulfide and mercaptan. Generally, it is believed that in poisoning by sulfur compounds, a reducing sulfur compound whose sulfur atoms per se can be coordinated at the active site provides stronger poisoning.
Further, the concentration of methane present in the coal mine ventilation gas varies widely from 0.1 to 1% and also prediction of its variation behavior is difficult. So, with simple use of a heat exchanger and a catalyst in combination, the gas temperature at the catalyst inlet will be reduced so that sufficient removal performance cannot be obtained. Conversely, in the event of sharp rise in the methane concentration, there occurs a sharp and sudden rise in the catalyst layer temperature within a short period of time, which leads to irreversible deterioration of catalyst activity. Especially, when there occurs a sharp rise in the methane concentration, the catalyst inlet temperature too will rise due to the effect of heat exchange, which in turn invites further rise in the catalyst layer temperature. As a result, there occurs a sharp rise in the catalyst layer temperature within a short period of time, which may invite fatal destruction of the catalyst and/or the heat exchanger. Also known is a method that uses heating by a burner in addition to a heat exchanger, so that when the preheating temperature (=catalyst inlet temperature) has risen above a predetermined value, combustion of the burner is stopped, thus stabilizing the catalyst inlet temperature (Patent Document 3). However, as this method requires fuel for the burner, the running cost increases. Further, the method involves a process in which as the methane concentration increases, the catalyst outlet temperature rises, which in turn leads to rise of the catalyst inlet temperature. Hence, there occurs a significant lag from the increase in methane concentration to the detection of rise in the catalyst inlet temperature. As a result, in the event of sudden and rapid variation in the methane concentration, deterioration of catalyst activity is unavoidable.